Immunologoc study of the rheumatoid like arthritis produced in mice by intravenous inoculation of Myocoplasma pulmonis has continued. Comparison was made of the presence of viable mycoplasma and mycoplasma antigen in infected mice with clinically arthritic and normal joints. Half of either arthritic or normal joints had no culturable mycoplasma but did contain antigen. This suggests that reaction to antigen does not play a role in pathogenesis. Viable mycoplasma were obtained more frequently (39%) from arthritic than normal (11.5%) joints. Antibody titer against M. pulmonis and normal mouse synovium were determined in serum and joint homogenate in arthritic mice. Both types of antibody were present in higher titer in joints than in serum, suggesting that antibody is either made locally or accumulates in the joint. A study was done of the effect of immunosuppression on the clinical course of the disease. Cyclophosphamide, 30 mg/kg twice weekly, reduced peripheral lymphocyte count to half to a third of infected untreated controls. However, when treatment was begun at 14 days, the peak of clinical disease, and continued for 155 days no effect on severity of arthritis was noted. This was judged by intensity of joint swelling and proportion of joints affected.